Power control in mobile radio telephone systems when transmission is interrupted

ABSTRACT

The invention relates to a method for controlling the transmission power in a communications system, especially in a mobile radio telepone system, comprising at least one mobile sition (UE) and at least one base station (NB) communicating therewith. The invention also relates to a communications system for carrying out said method. The aim of the invention is to allow for improved power control when the communications connection is interrupted. The receiving station uses a signal (BCH) for detecting power control instructions (TPC-Bit), whereby said signal is independent from communications data.

[0001] Power control in mobile radio systems during interrupted transmission

[0002] The invention relates to a method for transmission power control in a radio communications system, in particular in a mobile radio system.

[0003] In radio communications systems, such as the second generation European Mobile Radio System GSM (Global System for Mobile Communications), information such as speech, picture information or other data is transmitted by means of electromagnetic waves via a radio interface. The radio interface relates to a connection between a base station and a large number of subscriber stations, in which case the subscriber stations may, for example, be mobile stations or fixed-position radio stations. The electromagnetic waves are in this case transmitted at carrier frequencies which are in a frequency band provided for the respective system. Frequencies in the frequency band around 2000MHz have been provided for future radio communications systems, such as the UMTS (Universal Mobile Telecommunications system), or other 3^(rd) generation systems. Two modes are envisaged for the third mobile radio generation UMTS, with one mode being referred to as FDD operation (Frequency Division Duplex), and the other mode being referred to as TDD operation (Time Division Duplex). These modes are used in different frequency bands, with both modes supporting a so-called CDMA subscriber separation method (Code Division Multiple Access).

[0004] In mobile radio systems which use subscriber separation based on a CDMA method, rapid power control is necessary in order to ensure reliable reception of all existing communication links at the location of the base station, with few disturbances due to interference from adjacent transmission channels occurring at the same time. The rapid transmission power control is required in particular for real time services such as speech transmission at low speeds. Rapid transmission power control based on a closed control loop and an open control loop is implemented for the FDD mode according to the prior art, for example as in the document ARIB “Japan's Proposal for Candidate Radio Transmission Technology on IMT-2000: W-CDMA”, Jun. 1998, Japan, pages 39 to 42. The use of the open or closed control loop is in this case dependent on the respective type of transmission channel. The rapid closed control loop is based on so-called TPC bits (Transmitter Power Control), which are signaled periodically from the respective controlling device in the radio communications system to the other device. A base station thus controls the transmission power of the subscriber station, and vice versa.

[0005] A transmission power principle such as this is also in principle intended to be used for the TDD mode, for harmonization between the two modes in the UMTS mobile radio system.

[0006] In order to keep the interference in the TDD mode low, and to stabilize out fading effects in the reception power level, an open power control loop is used in the uplink direction UL and a closed power control loop is used in the downlink direction DL, with these generally being referred to as power control. This results in the inner and outer control loops in the downlink direction. The outer loop measures the quality of the input signal (for example frame error rate) and then presets a nominal value (SIR target) for the signal/interference ratio (SIR), using which the required quality can be provided. The inner loop measures the actual signal/interference ratio of the received signal, and compares this with the nominal value for the signal/interference ratio. Depending on the result, a power control command is transmitted to the opposite end, to change the transmission power level appropriately. The inner loop thus operates on a frame basis and very quickly, while the outer loop varies the nominal value only slowly. However, this type of power control is dependent on both transmission and reception taking place permanently. The following text considers in particular the power control process in the closed loop.

[0007] In many situations, for example when there is no data to be transmitted, for example in the case of discontinuous transmission (DTX) of radio frames, the transmission between one terminal, for example a mobile station, and a base station is, however, interrupted, at least in one direction. It is then either impossible to transmit a power control command, and/or no signal is available for reception measurements. The latter situation is important at the time at which the transmission is intended to be resumed, in which case, however, the transmission power level is then undefined, as a result of a lengthy interruption. Furthermore, in this situation without an input signal, both the inner power control loop and the outer power control loop are interrupted. The described problem occurs mainly in TDD systems, for example in UTRA-TDD, when the transmission there is interrupted at times in the event of discontinuous transmission, and the power control loop is hence also interrupted. In the case of CDMA systems using an FDD operating mode, this is generally not critical, since at least control information is still transmitted during an interruption in the data.

[0008] In the case of present-day systems, the transmission power has a safety supplement added to it after an interruption, that is to say transmission takes place at an increased intensity. However, this produces unnecessary interference.

[0009] The invention is based on the object of specifying a method which allows improved transmission power control during interrupted data transmission in a radio communications system.

[0010] This object is achieved by the method having the features of claim 1 and by the communications system having the features of claim 15. Advantageous developments are the subject matter of dependent claims.

[0011] An “external” signal is advantageously used in order to maintain the inner power control loop and the outer power control loop when there is no data signal. Power control commands are preferably transmitted unchanged to the transmitter in this case. Due to the lack of data or data signals to be transmitted, the transmitter knows that it must first of all accumulate the commands, and must then correct the transmission power by the accumulated value at a later stage.

[0012] In principle, the commands can be added up in the transmitter and in the receiver. If they are added up in the transmitter, the sum can be transmitted to the receiver at regular or irregular time intervals. The operation of the method is thus also possible, in particular, when the regular transmission of power control commands is also impossible.

[0013] If there is no power control command data input either, the already transmitted values are advantageously kept constant in the receiver.

[0014] Alternatively or in addition to this, it is also possible to form sums of the power control commands automatically in the receiver and/or in the transmitter, with the sum being transmitted with a time delay during transmission pauses in the uplink direction.

[0015] The sum is advantageously corrected using an “uncertainty factor” which is obtained, for example, from the number of power control commands which it has not been possible to decode.

[0016] Exemplary embodiments will be explained in more detail in the following text with reference to the drawing, in which:

[0017]FIG. 1 shows a block diagram of a radio communications system, in particular of a mobile radio system,

[0018]FIG. 2 shows a schematic illustration of the structure of a TDD radio interface, and

[0019]FIG. 3 shows a block diagram of an exemplary implementation of the inner and outer control loop in a transmitting/receiving device for a base station or subscriber station.

[0020]FIG. 1 shows a part of a mobile radio system as an example of the structure of a radio communications system. A mobile radio system in each case comprises a large number of mobile switching centers MSC, which are part of a switching network (SSS—Switching Subsystem) and are networked with one another, and/or produce access to a landline network, as well as in each case having one or more base station systems BSS (BSS—Base Station Subsystem), which are connected to the mobile switching centers MSC. A base station system BSS also has at least one device RNC (RNC—Radio Network Controller) for assigning radio resources, as well as at least one base station NB (NB—Node B) respectively connected thereto. A base station NB can set up connections via a radio interface to communication terminals or subscriber stations UE (UE—User Equipment), such as mobile stations or other types of mobile and stationary terminals. At least one radio cell Z is formed by each base station NB. The size of the radio cell Z is generally governed by the range of a general signaling channel (BCH—Broadcast Control Channel), which is transmitted by the base stations NB1, NB2 at a respectively defined and constant transmission power level. In the case of sectorization or in case of hierarchical cell structures, each base station NB may also supply a number of radio cells Z. The functionality of this structure can be transferred to other radio communications systems in which the development described in the following text can be used.

[0021] The example in FIG. 1 shows a subscriber station UE which is in the form of a mobile station and is moving, for example, at a velocity V in the radio cell Z of the base station NB. The subscriber station UE has set up a connection to the base station NB, on which signals for a selected service are being transmitted in the uplink direction UL and in the downlink direction EL. During the connection, the subscriber station UE periodically evaluates the transmission characteristics of the radio interface to the base station NB supplying it as well as to other base stations NB surrounding the subscriber station UE in order that, if the transmission quality deteriorates, it can request an increase in the transmission power level from the base station NB or, for example, can request that its connection be switched to an adjacent base station NB. An equivalent situation applies if the transmission quality improves, in which case the possibility to reduce the transmission power level is signaled in order to minimize the interference in the radio cell Z.

[0022] The subscriber station UE controls the transmission power level of the base station NB by means of signaling messages (TPC commands), in which, by way of example, it maps a measured variation of the transmission characteristics into a required transmission power change. The transmission characteristics are characterized by a value SIR and BER or its mean value BERavg averaged over a time interval. The periodicity of the outer control loop “Outer Loop” may be chosen, by way of example, as the time interval for the averaging process. Quality assessments such as the bit error rate or the frame error rate may be used, by way of example, as the characteristic value BER. Radio measurement variables such as the path loss, an interference situation at the location of the subscriber station UE as well as combinations of these parameters are preferably used for SIR. The variation of the characteristic values BER and SIR may also be determined additionally or alternatively on the basis of the general signaling channel, broadcast channel BCH, which is transmitted at a constant transmission power level that is known by the subscriber station. The base station NB carries out an identical procedure for controlling the transmission power level of the subscriber station UE. After appropriate evaluation of the transmission characteristics, it signals to the subscriber station UE any increase or decrease in the transmission power level. This signaling takes place, according to the prior art mentioned in the introduction for the FDD mode of the ARIB, for example by means of a specific TPC bit (Transmitter Power Control).

[0023] The frame structure for radio transmission in the TDD mode of the UMTS mobile radio system, in which the proposed method can advantageously be used, is shown in FIG. 2. A broad frequency band is subdivided on the basis of a TDMA component (Time Division Multiple Access) into a number of time slots ts with the same time duration, for example 16 time slots ts0 to ts15, which form a time frame fr. A frequency band B extends over a specific range of frequencies. Some of the time slots are used for signal transmission in the downlink direction DL, and some of the time slots are used for signal transmission in the uplink direction UL. An asymmetry ratio of 3:2 in favor of the downlink direction DL is illustrated, by way of example. With this TDD transmission method, the frequency band B for the uplink direction UL corresponds to the frequency band B for the downlink direction DL. The same situation is repeated further carrier frequencies. The variable assignment of the time slots ts for the uplink or downlink directions UL, DL allows a wide range of asymmetric resource assignments to be used.

[0024] Information from a number of connections is transmitted in radio blocks within the time slots ts. The data d is spread on a connection-specific basis with a fine structure, a spread code c, so that it is possible to separate, by way of example n connections by means of this CDMA component (Code Division Multiple Access) at the receiving end. The spreading of individual symbols of data d results in Q chips of duration T_(chip) being transmitted within the symbol duration T_(sym). The Q chips in this case form the connection-specific spread code C.

[0025] As will also be explained in the following text with reference to FIG. 3, the outer control loop Outer Loop is used in a method and a circuit not only for controlling the transmission power level of the base station NB but also for controlling the transmission power level of the subscriber station UE. A target value of a signal/interference ratio Target is determined and is in each case signaled in the outer control loop Outer Loop of the base station NB and of the subscriber station UE. The target value for the signal/interference ratio Target SIR ensures an adequate transmission quality, and must be matched as appropriate to the current transmission conditions. The respective inner control loop may in this case carry out particularly rapid transmission power control. This rapid transmission power control is carried out via the TPC bit, which in each case results in an increase or decrease in the transmission power level by a specific value in dB.

[0026] By way of example, FIG. 3 shows one implementation of the combination according to the invention of an inner control loop Inner Loop and an outer control loop Outer Loop in a transmitting/receiver device for the base station NB or else for the subscriber station UE. The structure corresponds to the structure in FIG. 3.5-5, page 40 of the known prior art from the ARIB. The received signals are filtered in a so-called matched filter, and are supplied to a detector device. In the illustrated example, the detector device is in form of a known RAKE combiner.

[0027] A signal-interference ratio SIR is determined in the inner control loop Inner Loop from the detected signals. The signal/interference ratio SIR is used for rapid transmission power control as the basis for optimum transmission power adjustment, since the interference situation at the receiver represents the most important criterion for reliable reception of the signals.

[0028] The received signals are decoded in an outer control loop in a decoding device, for example a Viterbi decoder, connected downstream from the detector device. A characteristic quality value BER is then in each case determined. An average of the characteristic value BER is in this case formed in the present circuit. This characteristic value BER is then compared in the outer control loop Outer Loop with a target value for the characteristic quality value Target BER, and the difference dBER between the two values is calculated. The difference dBER is then converted, for example by means of a weighting factor and other possibly non-linear operations, to a correction value dSIR for the signal/interference ratio. An addition process is then carried out with a target value for the signal/interference ratio SIR(j) from a preceding control interval j. The current target value resulting from this for the signal/interference ratio SIR(j+l) or for the Target SIR is delayed in the same manner by means of a delay device DELAY by in this case one j-step, and is then taken into account for the calculation of the subsequent target value. The current nominal value Target SIR is used for rapid transmissions power control in the inner control loop Inner Loop as the basis for increasing or reducing the transmission power level by the signaling of a TPC bit.

[0029] In order to improve the transmission power control during interrupted data transmission, the following text describes a method and an upgraded circuit, in which another channel with known characteristics is used for measurements and, furthermore, power control commands are still transmitted.

[0030] Interrupted data transmissions occur, by way of example, with UTRA-TDD, when discontinuous data transmission (DTX) is used for a downlink transmission, that is to say in the so-called downlink. During the interruptions, the subscriber station UE can no longer carry out any measurements on the data channel. In fact, there is always a common control channel in the downlink connection, for example, the signaling or broadcast channel BCH, which can be received by every subscriber station UE and which is neither directional nor subject to power control.

[0031] The subscriber station UE regularly measures the reception power level ELBC and the quality QBC of the broadcast channel BCH and, in addition, the disturbance power level of the data. As a rule, it can be assumed that this reception power level ELBC on the broadcast channel BCH, in particular, fluctuates relative to the reception quality of data which is received on a data channel. Known algorithms can thus still be used for controlling the inner loop for power control of the transmission pauses, since the supply can be maintained using measurement values from another measurement.

[0032] During pauses in the data transmission in the downlink direction, the subscriber station UE uses in its inner loop the i-th virtual signal/interference ratio value:

SIR _(virt)(i)=RSCP _(virt)(i)−ISCP(i).

[0033] The index i starts from 1 for the data transmission frames during an interruption or transmission pause.

[0034] RSCP_(virt)(i) represents the virtual received signal power level, and ISCP(i) represents the interference signal power level in the associated data channel. In this case, furthermore

RSCP _(virt)(i)=RSCP(0)+L(0)−L(i)+ΣTPC(k).

[0035] L(0) represents the last path loss value before the transmission pause, and L(i) represents the current path loss. The path loss is obtained from the difference between the known transmission power level of the broadcast channel BCH and its measured reception power level. All the parameters in the above formulae are logarithmic, that is to say they can be regarded as being in dB or dBm. The value RSCP_(virt) (i) thus takes into account the most recent actually measured RSCP value before the interruption, the path loss L(i) in dB which was measured on the broadcast channel BCH, the path loss L(0) determined most recently before the interruption, and the sum of the power control commands TPC(k) in dB over i−1 frames during the interruption. In the case of a TPC bit with only two logic states 0 and 1, it is possible, by way of example, to use the TPC bit “1” for an increase and the TPC bit “0” for a decrease by one step of magnitude x dB, that is to say TPC=+x or −x, respectively.

[0036] During interruptions, the TPC commands are calculated by means of these virtual signal/interference ratio measurements, which are based, by way of example, on the measurement of the reception power level of the broadcast channel BCH, in the same way as would otherwise be done by means of the actual signal/interference ratio measurements in the case of an active data channel.

[0037] In the example of the circuit sketched in FIG. 3, signals from the broadcast channel BCH are investigated in the detector device, in the event of an interruption in the data signal, in a corresponding way instead of signals from the data channel.

[0038] It is also possible for the calculation of the power control commands in the UE to take account of the fact that there is a delay representing a number of time slots between the time slot for the broadcast channel BCH and the time slot for the data channel. This delay can result in an estimate which is based on the broadcast channel time slot having a low confidence factor for the data time slot. Longer-term time averaging of the measurement values on the broadcast channel BCH may therefore be carried out, depending on the delay. This means that, if there is a long delay, the current measurement value is given a low weighting in comparison to this mean value while, if the delay is short, the current measurement value is given a stronger weighting. Since the uplink and downlink in a TDD system are reciprocal, the nature of the weighting can also be transferred from the weighting as is used, for example, for the open power control loop in the uplink direction.

[0039] An example of the profile of the reception power level of the broadcast channel BCH can be described as follows. If no more data is being received, the inner and the outer power control loops are maintained using measurements on the broadcast channel BCH. These measurements are interpreted relative to the quality Qo (for example the bit error rate), the reception power level ELBC of the broadcast channel BCH and the disturbance power level To immediately before the interruption in the data transmission. If the signal/interference ratio SIR of the most recently received signal section of the data differs from the signal/interference ratio nominal value SIR-target, then this is taken into account by means of an appropriate correction, with the most recent “correct” power control command preferably being taken into account in the method and algorithm described in the following text. The inner loop now measures the difference between the reception power level of the broadcast channel BCH and the most recently measured power level Po. This and the further measured disturbance power level which would occur if data were still present are used to calculate a virtual signal/interference ratio value STRvirt, which is compared with the most recently calculated signal/interference ratio nominal value SIR-Target for data. Depending on the power control stepwidth, any change in the difference leads to power control commands “TPC Commands” which are transmitted to the transmitter or to the base station NB.

[0040] In the example of a circuit as illustrated in FIG. 3, signals from the broadcast channel BCCHBCH are investigated in the detector device in the event of an interruption in the data signal, in a corresponding manner, instead of signals from the data channel.

[0041] The following text is based on a control system example with an assumed stepwidth of 1 dB. Initially, the reception power level rises continuously by 10 dB at a rate of 1 dB/frame, then by 20 dB at 2 dB/frame, before then remaining constant at Po=+30 dB. The disturbance power level over the first 15 frames is constant, and then rises by 2 dB per frame to Io=−10 dB. 20 commands to reduce the transmission power level by 1 dB are sent successively from the mobile station UE, followed by commands to leave the transmission power level constant.

[0042] If the command for a constant transmission power level in the communications system or in one of the stations involved should not be implemented, a higher and a lower transmission power level can be signaled alternatively. The sum S or ΔO of the transmitted power control commands thus follows the difference between SIR_(virt) relative to SIR_(target).

[0043] In one preferred embodiment, the TPC commands received in the meantime are added up in the transmitter, and the sum is stored. After a transmission pause, the first signal section of the data is transmitted at the power level S*stepwidth+‘most recent transmission power level before the transmission pause. In some circumstances, the transmission power level may also be corrected, if the rate matching has changed since the last transmission. Furthermore, the transmission power level also has an externally adjustable correction term applied to it.

[0044] In one preferred embodiment, the TPC commands received in the meantime are advantageously added up in the correction term as well, and the sum is stored. After a transmission pause, the first signal section of the data is transmitted at the power level S*stepwidth+‘most recent transmission power level before the transmission pause. In some circumstances, the transmission power level may also be corrected, if the rate matching has changed since the last transmission. Furthermore, the transmission power level also has an externally adjustable correction term applied to it.

[0045] The correction term also advantageously takes account of any change in the rate matching and/or channel coding. If, by way of example, less data is transmitted in a frame, then this data can be better protected, resulting in better quality after the decoding process, for the same SIR.

[0046] Errors can occur in the transmission of the TPC commands from the mobile station UE to the base station NB, so that the base station NB may not be able to read or decode the commands. This results in uncertainties in the sum calculation ΣTPC(k). However, it can be assumed that, statistically, both “higher” and “lower” commands are lost, so that the effects are cancelled out. Furthermore, the receiver can take account of a certainty supplement or safety deduction in the transmission power level, depending on the number of commands which cannot be decoded.

[0047] If the transmission is interrupted in the uplink direction, this results in a situation in which, although the mobile station UE can carry out all the measurements, it cannot, however, transmit the power control commands that are produced to the network (node B). In this situation, one embodiment provides for the mobile station UE to add up all the generated power control commands TPC. The running sum may be transmitted either periodically or on the basis of a specific stimulus, for example a threshold being exceeded, via, for example, channels at a higher level to the base station NB. Alternatively or additionally, it is possible to provide for the sum to be transmitted to the base station NB after the resumption of data transmission. Alternatively, instead of the sum, the difference between SIR_(target) and SIR or SIRvirt at the transmission time may also be transmitted.

[0048] In addition to the inner control loop described above, the outer control loop can also continue to run. The outer control loop detects differences which cannot be regulated out by the signal/interference ratio SIR. By way of example, with the same signal/interference ratio SIR, the quality of a signal may differ if the transmission channel or, for example, the velocity of the mobile station UE changes. In situations such as this, the outer control loop has to follow the nominal value SIR_(target) of the inner control loop. In order to ensure this mechanism takes place even with measurements on the broadcast channel BCH, the quality of the BCH must take account of the difference between the current signal/interference ratio on the broadcast channel (SIRbc(i)) and its initial value (SIRbc(0)).

[0049] The outer control loop takes the initial quality value of the broadcast channel BCH at the start of the transmission pause as a nominal variable, and compares the current quality value of the broadcast channel BCH with this, in order in this way to derive a nominal value SIR-target for the signal/interference ratio SIR for the inner control loop. This process should also take account of the fact that the signal/interference ratio SIR of the broadcast channel BCH may have changed, and its quality may then also have changed. This effect on the quality of the signal of the broadcast channel BCH should be compensated for in an appropriate manner. If the signal/interference ratio SIR on the broadcast channel BCH differs from the initial value by, by way of example, 3 dB in the upward direction, then the quality Q before the comparison with the initial quality value Qo is corrected using the value dQ, which is obtained from a signal/interference ratio SIR which is 3 dB too high. This compensation process decouples the influence of the signal/interference ratio SIR from the influence of the channel. This means that if, after the correction by dQ, there is still a difference between the initial quality value Qo and the correct broadcast channel quality value QBC+dQ, then this difference is due purely to changes in the transmission channel.

[0050] The association of a correction value dQ with a difference in the signal/interference ratio SIR may be read from a table or matrix. The entries in the matrix may be derived automatically from previous empirical values, or may be predetermined in a fixed manner. 

1. A method for transmission power control in a radio communications system having at least one first and one second station (UE, NB) which communicate with one another at least at times, in which a receiving one of the two stations (UE, NB) determines the reception power of a signal which is sent from the other station (NB, UE), and the transmitting station (NB) transmits a power control instruction (TPC bit), characterized in that the receiving station uses a signal (BCH) which is independent of communication data in order to determine the power control instruction (TPC bit).
 2. The method as claimed in claim 1, in which the power control instruction is determined during active communication by means of a signal which is dependent on communication data.
 3. The method as claimed in claim 1 or 2, in which the power control instruction is determined during interrupted communication by means of the signal which is independent of communication data.
 4. The method as claimed in one of the preceding claims, in which the communication method supports a TDD or FDD method.
 5. The method as claimed in one of the preceding claims, in which the receiving station (UE) is a communication terminal, in particular a mobile station, which transmits the power control instruction (TPC bit) in an uplink connection (UL) to the transmitting station (NB), in particular a base station.
 6. The method as claimed in one of the preceding claims, in which power control instructions (TPC bits) are transmitted during existing and interrupted data communications.
 7. The method as claimed in one of the preceding claims, in which the values of a large number of individual power control instructions (TPC bits) are added up in the receiving and/or transmitting station (UE, NB) to form a sum as a power control instruction.
 8. The method as claimed in claim 7, in which the sum is used as power control instructions for power control.
 9. The method as claimed in claim 7 or 8, in which the sum is transmitted at regular or irregular intervals or before the resumption of a communications connection to the transmitting station.
 10. The method as claimed in one of the preceding claims, in which the power control instruction (TPC bit) is corrected by means of a correction value.
 11. The method as claimed in claim 10, in which the correction value represents a safety supplement or safety deduction based on the, in particular estimated, detection certainty of the received power control instructions (TPC bits).
 12. The method as claimed in claim 10 or 11, in which the correction value takes account of changed rate matching and/or channel coding.
 13. The method as claimed in one of the preceding claims, in which a weighting between the current path loss and a long-term path loss is taken into account when producing the power control instructions.
 14. The method as claimed in one of the preceding claims, in which the power control instructions (TPC bits) are processed in an inner control loop in the transmitting station.
 15. A radio communications system having at least one first and one second station (UE, NB), which communicate with one another, for carrying out the method as claimed in claim
 1. 16. A communication station for a radio communications system as claimed in claim 15 and/or for carrying the method as claimed in claim 1, with the communication station being a communication terminal, in particular a mobile station (UE) or a base station (NB). 